In general, electrochromism refers to a phenomenon where a color is changed by oxidation/reduction reactions according to the potential of the electric field applied from the outside. Electrochromic materials are classified into organic material and inorganic material. The organic material includes viologen, anthraquinone, phenothiazine, polyaniline, polypyrrole, polythiophene, etc. The inorganic material includes a reductive electrochromic material such as WO3, MoO3, CeO2, MnO2, Nb2O5, etc., and an oxidative electrochromic material such as Ir(OH)x, NiO, LiNiO2, etc.
In the case of the inorganic electrochromic material used for most oxidative electrodes, in preparing an electrochromic material, a sol-gel reaction is performed only at temperatures of 400° C. or more. In addition, there is a problem in that a device requires a high operating voltage, a long coloring/decoloring response time is required, and optical properties are not enough. On the other hand, lithium nickel oxide (LixNi1-yO), which is an electrochromic material used for an oxidative electrode, has advantages including high light transmittance, excellent electrochromic properties, low voltage driving ability, a high response speed, etc. Therefore, research on such lithium nickel oxide has been recently performed. Especially, on the structure of lithium nickel oxide (LixNi1-yO) prepared by a sol-gel method, a mechanism on intercalation and deintercalation of Li+, and the change in the structure, much research has already been conducted.
Lithium nickel oxide (LixNi1-yO) is generally prepared by a sol-gel method, a sputtering method, a pulsed laser deposition method, etc. However, when the lithium nickel oxide (LixNi1-yO) formed on a conductive substrate is used as an electrochromic material, some problems exist.
A lithium nickel oxide (LixNi1-yO) electrode, in which a lithium nickel oxide layer is deposited by a sputtering method, is in an unstable state where both Ni2+ and Ni3+ exist in the structure. Accordingly, in an electrochromic device using the lithium nickel oxide (LixNi1-yO) there are some serious problems in that the electrochromic range is narrowed due to incomplete oxidation/reduction of lithium nickel oxide (LixNi1-yO) and especially, the device does not become colorless and transparent due to incomplete reduction of lithium nickel oxide. Therefore, when an electrochromic device is obtained by cohering an oxidative electrode including a lithium nickel oxide (LixNi1-yO) layer with a reductive electrode, lithium nickel oxide (LixNi1-yO) is required to be completely decolored by repeatedly applying oxidation/reduction voltages to the electrochromic device.
In the case of a lithium nickel oxide (LixNi1-yO) layer having a thickness of about 150 nm or more, lithium nickel oxide (LixNi1-yO) is decolored by repeatedly applying oxidation/reduction voltages several hundred times. However, herein, there is a problem in that an electrochromic device is incompletely decolored.
Also, a lithium nickel oxide (LixNi1-yO) material, which is deposited by a sputtering method, exhibits poor adhesion onto the surface of FTO(Fluorine-doped Tin Oxide) glass. Accordingly, when oxidation/reduction is repeatedly performed on an electrochromic device, an electrode material is changed into NiOx or NiOOH, and thus may be stripped from an electrode surface or cause bubbles within electrolyte.